1. Field of Invention
This invention relates to an aggregate of sand, gravel and asphalt useful for road base or repair material. More particularly, the present invention pertains to compositions of asphalt with aggregate, which further include recycled tire materials and thermoplastics.
2. Prior Art
The challenge of preparing a road surface which is capable of weathering extreme variations in temperature has been and remains formidable. When one considers the fact that ranges in temperature in typical environments can traverse 100 degrees centigrade, with the attendant expansion and freezing of water, it is not surprising that road repair is a major budget item for virtually every state.
Black top or asphalt road base is particularly vulnerable because of the tendency for poor compaction between particles of aggregate. This occurs in part because the nonuniform size and shape of gravel aggregate leaves spaces between aggregate particles which do not totally fill with sand or asphalt binder. Additionally, the intermediate aggregate which functions to fill in spaces between gravel may be nonuniform in size, thereby frustrating good compaction. Water is therefore able to seep into these areas, and upon freezing, expand to open cracks and gaps in the material. The process then repeats itself, further enlarging the cracks and fissures until fragments of pavement break away, leaving pot holes and other road damage.
Recent awareness for the need to recycle discarded tires has prompted their use as an additional composite element to the asphalt aggregate combination. This element comprises shredded tire which is softened or dissolved and serves as a binder with the bitumen component. Examples of this technology are disclosed in U.S. Pat. Nos. 3,253,521; 3,567,660; and 4,166,049. These references teach the benefit of recycling tire material within a road base, but don't indicate any strong advantage associated with prevention of weathering of the road material. The introduction of shredded tire material within the asphalt composite functions more as an ecological solution for disposition of worthless tires, rather than as part of a solution to improve the road base.
Another application of discarded tires has developed in connection with preparation of an intermediate layer which is designed to provide a resilient function to limit damage to the top layer of asphalt composite. This intermediate layer is typically formed of tire material supported in sand. There is no significant relationship of the tire with the bitumen material in these references, which are represented by U.S. Pat. Nos. 3,707,901 and 3,850,537.
U.S. Pat. No. 4,358,320 illustrates another aspect of asphalt with aggregate in which the bitumen is used to enhance adhesive bonding of silicone or fiber reinforcement as part of a composite. In this instance, the structure of the aggregate is not at issue. Instead, the problem being solved relates to strengthening existing bonding between the filler and bitumen. It does not relate to compaction.
Despite the numerous attempts to find a useful approach for disposing of the millions of pounds of scrape rubber generated by the tire industry, the problem is simply compounding. Ecological concerns are increasing pressure on governments to deal with this disposal problem. This is also true with the increasing use of plastics which are non-biodegradable and have long been recognized as a landfill problem. Nevertheless, a practical solution must include development of a range of benefits that provide financial return, as well logical justification to go to the expense necessary to implement constructive use of recycled tire product in large scale.
The urgency of this problem has prompted the US Government to pass laws in 1991 requiring that at least 5% of the asphalt road composite comprise recycled tire material. This injunction mandating use of tire rubber will undoubtedly enhance commercial interest in techniques for combining these materials, but will not ensure an improved road structure. The fact that in 1966 U.S. Pat. No. 3,253,521 issued teaching multiple techniques for combining asphalt and tire material as a paving composition, evidences that the solution to an effective composition is not a simple matter. If incorporating tire material within asphalt were merely a step of stirring it in, the past 26 years would have seen greater acceptance. The existence of a law requiring this use of tire with asphalt confirms that prior art commercial procedures have failed to develop a product which is an improvement over other prior art paving compositions.
It is interesting to note that the 1966 patent identified the significant issues in preparing this asphalt composite to include the proper selection of particle size for the aggregate and tire scrap. Column 3, lines 18-29 suggest that the rubber particles should have a range of sizes extending from screen size 4 through 80 or 0.01 to 0.25 inches. The range of sizes was suggested to ensure that voids within the paving material would be minimized. This material was then to be formed into blocks which could be laid as a paving composition.
This reference did not contemplate use with mineral aggregate, but appears to be concentrating on the development of a new form of paving composition which is primarily formed of the rubberized bitumen and particulated rubber. This appears to be true generally with the other cited patents, in that they emphasize the formation of a composition which is as much as 20 to 60% rubber composition (U.S. Pat. No. 4,166,049). U.S. Pat. No. 3,707,901 suggests as much as 30 to 60% (volume) vulcanized rubber with an equal amount of mineral aggregate. Clearly, the emphasis has been to load the asphalt paving composition with as much tire material as can be chemically introduced while maintaining acceptable physical properties. Experience suggests that the physical performance of these compositions did not prove adequate. Deformation under heat with subsequent formation of waves in the road surface occurred where excessive organic binder becomes too flowable, perhaps because of the high rubber content. Weatherability of the prior art composites remains a problem.
What is needed, therefore, is a form of paving material which preserves the desirable properties of an asphalt aggregate without compromising those properties for the sake of resolving the ecological problem of scrap tires.